This invention relates to a control of an infinitely variable transmission of a vehicle.
Tokkai 2000-179674 published by the Japanese Patent Office in 2000 discloses an infinitely variable transmission (IVT) in which a continuously variable transmission (CVT), a fixed speed ratio transmission and a planetary gear set are combined.
The speed ratio of the IVT (IVT speed ratio) arbitrarily varies from forward to reverse, including a geared neutral point (GNP) at which the speed ratio becomes infinite, by changing the speed ratio of the CVT (CVT speed ratio).
At GNP, the output shaft of the IVT stops. In other words, only the variation in the CVT speed ratio allows change-over of forward and reverse without using a forward/reverse change-over mechanism.
In this IVT, the CVT speed ratio is controlled based on the accelerator pedal depression and vehicle speed. Even when the accelerator pedal is not depressed, a small torque is transmitted to the drive wheels of the vehicle to cause the vehicle to creep, i.e., to make the vehicle move at a low speed. The creep torque transmitted to the drive wheels in this state is obtained through the CVT speed ratio control wherein the CVT speed ratio is feed-back controlled so that the real vehicle speed detected by the vehicle speed sensor coincides with the target vehicle creep speed.
As a result, when a driver depresses the brake pedal in order to stop the vehicle at low speed, the difference between the real vehicle speed and the target vehicle creep speed increases and the CVT speed ratio is controlled to increase the creep torque transmitted to the drive wheels in order to achieve the target vehicle creep speed.
This increase in the creep torque is opposite to the intention of the driver who operates the brake to stop the vehicle, and the driver may experience an uncomfortable feeling. Further, IVT has the following output torque characteristics by its nature. When the CVT speed ratio corresponds to GNP, the torque transmitted from IVT to the drive wheels is zero. When the CVT speed ratio is slightly increased from the GNP, the output torque of the IVT increases abruptly and largely, and then it gradually decreases as the CVT speed ratio further increases. Such an abrupt increase in the output torque of the IVT may adversely affect the smooth starting of the vehicle.
It is therefore an object of this invention to optimize the creep torque of a vehicle equipped with an IVT through the speed ratio control thereof.
In order to achieve the above object, this invention provides a control device for an infinitely variable transmission for a vehicle that comprises an input shaft, a continuously variable transmission which outputs the rotation of the input shaft at an arbitrary speed ratio, a fixed speed ratio transmission which outputs the rotation of the input shaft at a fixed speed ratio, and an output shaft which changes a rotation direction and a rotation speed according to a difference between an output rotation speed of the continuously variable transmission and an output rotation speed. The control device comprises a sensor which detects a running state of the vehicle, a sensor which detects a real vehicle speed, and a programmable controller.
The programmable controller is programmed to calculate a target speed ratio of the continuously variable transmission based on the running state of the vehicle, set a target vehicle speed based on the running state of the vehicle, determine if a predetermined creep torque control condition holds based on the running state of the vehicle, limit the target vehicle speed, if the predetermined condition holds, to cause a difference between the target vehicle speed and the real vehicle speed to be within a predetermined range, calculate a corrected target speed ratio by correcting the target speed ratio to cause a difference between a limited target vehicle speed and the real vehicle speed to become smaller, and control the speed ratio of the continuously variable transmission based on the corrected target speed ratio.
The programmable controller may alternatively be programmed to calculate a target speed ratio of the continuously variable transmission based on the running state of the vehicle, set a target vehicle speed based on the running state of the vehicle, determine if a predetermined creep torque control condition holds based on the running state of the vehicle, calculate, if the predetermined condition holds, a corrected target speed ratio by correcting the target speed ratio to cause a ratio of a rotation speed of the input shaft and a rotation speed of the output shaft to become smaller as the target vehicle speed becomes greater, and control the speed ratio of the continuously variable transmission based on the corrected target speed ratio.
This invention also provides a control method for the above described infinitely variable transmission. The control method comprises detecting a running state of the vehicle, detecting a real vehicle speed, calculating a target speed ratio of the continuously variable transmission based on the running state of the vehicle, setting a target vehicle speed based on the running state of the vehicle, determining if a predetermined creep torque control condition holds based on the running state of the vehicle, limiting the target vehicle speed, if the predetermined condition holds, to cause a difference between the target vehicle speed and the real vehicle speed to be within a predetermined range, calculating a corrected target speed ratio by correcting the target speed ratio to cause a difference between a limited target vehicle speed and the real vehicle speed to become smaller, and controlling the speed ratio of the continuously variable transmission based on the corrected target speed ratio.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.